1. Field of Invention
This invention relates to microporous films. More particularly, this invention relates to microporous films which are produced by a stretching and heat setting process.
2. Prior Art
The preparation of microporous films, i.e. films having a pore size in the range of about less than about 0.1 microns has been accomplished by a wide variety of methods in the prior art.
One method has involved the physical penetration of a precursor film such as by needle punching or by treatment with chemical agents or radiation.
A more common method of preparing microporous films has involved the addition of a leachable agent to a polymer melt. The film is then formed and the leachant is dissolved out of the film to leave behind a porous product. The disadvantage of this process, among others, is that it requires that polymeric films be treated with materials which diminish the properties of the final films, if they are not totally dissolved from the film. Leached microporous films are therefore often "contaminated" by the presence of the leachant.
U.S. Pat. No. 4,247,478 is typical of the prior art patents which have involved leaching of a material from a precursor film to form a microporous film. According to this patent, a thermoplastic polymer is dissolved in a compatible liquid, as defined in the patent, the solution is then allowed to assume the desired shape and is cooled under certain specified conditions. Thereafter, a portion of the compatible liquid is removed to form a microporous film.
U.S. Pat. Nos. 3,382,306; 3,558,764; and 3,679,538 disclose the preparation of a microporous film formed from a polyolefin, polyacetal, polyethylene sulfide, etc. Basically the process in forming these films involves the steps of melt extrusion under high stress, annealing, cold drawing, hot drawing and then heat setting. According to the process of these patents, melt extrusion under high stress serves to form a row nucleated precursor film the superstructure of which is further perfected by annealing. The resulting film is then drawn and heat set. While the products produced from these films have excellent properties, the resulting films are much larger in pore size than the films of the instant invention. Moreover, the "pores" of the films described in the above patents are more like slits than they are like true pores. Also, the process of these patents does not permit the manufacture of films having extremely large thicknesses because it is difficult to produce completely row nucleated products with thicker films. Finally, the films prepared according to these patents are extremely weak in the transverse direction.
U.S. Pat. No. 3,250,146 appears to disclose the biaxial treatment of various types of films. See also U.S. Pat. No. 2,494,334, U.S. Pat. No. 4,076,785, and U.S. Pat. No. 2,494,334. However, none of the films described in these patents are microporous in nature.
Accordingly, it is an object of this invention to prepare microporous films which are of greater thickness than those prepared in the prior art.
It is another object of this invention to prepare microporous films which are not contaminated with leachants or other materials.
It is another object of this invention to prepare microporous films which have uniform pore size distributions.
It is yet another object of this invention to prepare microporous films having pores which are generally circular in nature and which pores are extremely small in diameter.
These and other objectives are attained according to the process of the instant invention.